The overall goals of this application are to establish the natural history of cyst progression prior to the development of renal insufficiency and to identify imaging and biochemical parameters which can be used as surrogate markers of disease activity of progression in polycystic kidney disease. Specific aims include: 1) Determine the rate of change of renal cysts and parenchyma volumes and of renal blood flow, their dependency on age, gender, ethnic background, and PKD genotype, and their correlation with other possible biochemical markers of disease activity or progression in prepubertal, pubertal and young adult patients with ADPKD (7-20 years) diagnosed for the purpose of the study and in non- affected sibling. 2) Determine the predictive value of absolute and relative renal cyst and parenchyma volumes, of renal blood flow, and of their changes over time, and of biochemical markers of disease activity for the rate of decline of renal function measured by serum cyst cystatin C levels and non-radiolabeled iothalamate clearance in patients with ADPKD and normal serum creatinine at the onset of the study. 3) Determine the rate of change of renal cyst and parenchyma volumes, renal blood flow, and biochemical markers of disease activity associated with pregnancy in ADPKD. 4) Study the renal cyst and parenchyma volumes, renal blood flow, and biochemical markers of disease activity associated with pregnancy in ADPKD. 4) Study the renal cyst and parenchyma v and parenchyma volumes, renal blood flow, liver imaging parameters, and biochemical markers of disease activity or progression in children with symptomatic early onset ADPKD or ARPKD and variable degrees of renal dysfunction. 5) Study the mechanical properties of the renal and hepatic parenchyma in ADPKD and ARPKD, using MR elastography. We propose to use innovative imaging studies based on state-of-the-art MR technology and 3d ultrasound. We anticipate that the results of this study will provide further insight into the pathogenesis of polycystic kidney disease and will facilitate the design of future intervention trials, shortening the observation period necessary to determine efficacy.